1. Field of the Invention
The present invention relates to the production of porous non-friable films from emulsion polymers, porous films formed at ambient temperature and processes of manufacturing films having permanent porosity.
2. Description of the Related Art
It is well known that latex films containing particles that are well ordered prior to the final stage of film formation exhibit the best barrier properties. When latex films are used as binders, the same barrier properties can be a disadvantage where access to reactive or adsorptive sites is required. Therefore, latex films having porous structures or transport pores are desirable would have utility in chemical and biochemical processes, such as those that utilize liquid barrier technology, breathable coatings, supported catalysts, sensor technology, encapsulated biocides, immobilized bacteria technology or immobilized cell technology.
A number of different techniques have been employed to create porous films by deliberate reductions in latex stability prior to film formation and by exceeding critical pigment volume fractions, the latter involves adding excess pigment or filler so that there is enough binder to glue the particles together yet not enough to completely fill interstitial voids, but each technique has serious drawbacks, namely, the inability to control and retain a pore structure in the film.
Compositions derived from an intimate mixture of an aqueous latex of a film forming or coating polymer and the cells of an organism have been disclosed in a European publication, EP 0 288,203 B1. The polymer has sufficient fluidity to undergo at least partial coalescence and a process for the production of an enzyme reaction product, formed by mixing the polymer, bacterial cells and a flocculant, causing the cells and the polymer to agglomerate. One important aspect of the process disclosed is the use of polymer flocculation to produce the porous polymer/bacteria agglomerates. This aspect limits the general utility of the process by requiring a second ingredient be added or some other trigger be used at the point of creating the porous agglomerates. A second aspect of this disclosure is the need to anneal the latex particles at a temperature above the Tg of the polymer. If the operating temperature of the porous agglomerates is at or above room temperature then the latex particle must be annealed substantially above room temperature. If the particles could be annealed at room temperature then the porous agglomerates would quickly lose porosity at room temperature. Another important limitation of the disclosure in EP 0 288,203 B1 is the inability of the porous agglomerates to function at high operating temperatures (T˜80° C.), due to continued particle coalescence. Polymers having relatively high Tg (>80° C.) in such a process, however, would require annealing at temperatures well above 80° C. to achieve sufficient fluidity, a condition which would be detrimental to the bacteria or other organisms. The additional requirement of relatively high operating temperatures has become more important as bio-processing technology has focused on thermophilic bacteria, which are capable of surviving at 80° C. for extended periods of time. It is clear that a process for forming smooth, porous films at ambient temperature, which are a capable of withstanding high operating temperatures without the concomitant loss of porosity is highly desirable, yet is not possible given the disclosure of EP 0 288,203 B1.
Another process for preparing porous composite membranes for ultrafiltration and micro-filtration membranes has been disclosed in European publication, EP 0 711,199 B1. The membranes are prepared by depositing discrete, spherical, polymeric particles, obtained by suspension, dispersion or emulsion polymerization on the surface of a porous substrate to obtain the composite and using thermal coalescence of the particles or chemical means to stabilize the resulting composite. A key limitation of the disclosure in EP 0 711,199 B1 is the need to thermally coalesce the latex particles at relatively high temperatures (>120° C.). There are many applications, including bacteria/latex composite films, in which the high annealing temperature is not practical. A number of typical processing and performance limitations associated with this membrane technology, such as the restricted choices of available pore sizes, has been detailed in a publication of Jons, Ries and McDonald in the Journal of Membrane Science, Vol. 155, pages 79-99 (1999). Thus, an enabling process to form porous films comprising latex particles at ambient temperature would indeed have significant utility.
Current aqueous latex polymer technology utilizes the process of latex film formation to afford continuous, non-porous films. In a number of important chemical and biochemical processes, however, polymer films that retain a high degree of porosity so as to allow small molecules to diffuse, relatively unhindered, in and out of the film are of great commercial utility. It is also desirable in such applications that film formation be accomplished at or close to ambient temperature and the resulting porous film not be friable after film formation is complete. A long recognized problem has been to make a permanently porous film from water-borne latex dispersion polymers, such that film formation occurs at ambient temperature and the resulting film once formed is not friable, possesses a high degree of porosity and retains porosity at elevated temperatures for long periods of time. Currently aqueous latex technology either affords films with no porosity, partially coalesced films that are non-uniform and have stability issues, films with high porosity which require elevated temperature for film formation, or films having high porosity which require polymer flocculation to create the porous structure.